In the installation of plaster board walls and ceilings it is conventional practice to apply a layer of hardenable mastic across the joint of adjacent wall board panels, employing apparatus as exemplified by various patents to R. G. Ames, U.S. Pat. Nos. 2,824,442 and 2,984,857 being particularly pertinent. Briefly, these patents disclose a tool comprising a mastic housing or reservoir with a discharge slot therein and a trailing trowelling blade which forms a uniform layer of mastic over the joint. The blade is slightly concave and sliding wear shoes adjacent its ends form two supports for the apparatus so that the cross section of the mastic is slightly convex and feathered at the ends of the blade. The "third" support for the apparatus comprises a pivoted axle at the leading end of the apparatus having a roller at each end thereof which rolls on the plaster board. While this is actually a four point support, it is, in effect, a three point support since the axle axis remains parallel to the plaster board surface and is equivalent to placing a single roller thereat.
In the operation of the apparatus, the wear plates and rollers are placed in engagement with the plaster board and pressure is applied by a handle pivotally adjustably connected to a swingable wall or plate which forces mastic through the slot and also moves the apparatus along the plaster board. When the rollers are ahead of the mastic being applied their peripheries remain free of accumulation of mastic. The direction of movement of the apparatus must be reversed, however, when it approaches the end of a joint and the rollers must now move toward the soft mastic layer which has just been applied. During this operation, they often roll over tacky mastic which adheres to their peripheral surfaces. Also, a layer of mastic must sometimes be applied across a previous layer in which event the rollers must move across same, similarly effecting transfer of mastic to their peripheries.
When accumulation of mastic on the rollers occurs, they become eccentric or bumpy, destroying their normal smooth rolling contact with the board, and vibrations are transmitted to the axle axis, which, in turn, are reflected at the trowelling bar, producing transverse lines or other irregularities in the mastic layer. To correct this, the apparatus has been removed from the board and the accumulation removed from the rollers with a finger nail or other scraping implement, entailing loss of time.